The present invention relates to electrophotographic processes. More specifically, the present invention relates to halftone color screening techniques for extending the range of relatively high contrast electrophotographic processes such as color xerography.
In the process of electrophotographic printing, as described in U.S. Pat. No. 2,297,691 issued to Carlson in 1942, the photoconductive surface has an electrostatic latent image recorded thereon. In the usual method of carrying out the process, the photoconductive surface is electrostatically charged substantially uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the irradiated areas of the charged photoconductive surface. The undischarged areas of the photoconductive surface thus form an electrostatic latent image in conformity with the configuration of the original light pattern.
The electrostatic latent image can then be developed by contacting it with a finely divided electrostatically attractable material such as a toner powder. The powder is held in image areas by the electrostatic field pattern on the photoconductive surface. Where the field is least, little or no material is deposited. Thus, a powder image is produced in conformity with a light image of the copy being reproduced. The powder is subsequently transferred to a sheet of paper, plastic, or any other suitable surface and affixed thereto to form a permanent print.
This basic process of electrophotographic printing may be utilized to produce color reproductions by slightly altering the aforementioned basic steps as, for example, disclosed in U.S. Pat. No. 3,531,195. Such color reproductions are accomplished by repeating the usual techniques of (1) charging the photoconductive surface, (2) exposing it to one color component of the subject matter to be reproduced, (3) developing the photoconductive surface with the appropriate toner, (4) transferring the toner image onto a support material in a selected repeated cycles of operations. The number of times the basic sequence for color reproduction is repeated depends on the number of colors utilized to develop the image. During each cycle a different color separation filter is utilized to selectively absorb and thereby transmit light of selected wavelengths to the photoconductive surface whereupon the photoconductive surface is discharged to a proper level. After the first exposure, using a first separation filter, the electrostatic latent image on the photoconductive surface is then developed by a complementary first color toner. The developed image of the first particular color is then transferred to a support material. The support material retaining the developed image of the first color is brought into position to receive images subsequently developed with other colored toners.
After transferring the developed image of the first color, the photoconductive surface is then cleaned and uniformly charged in preparation to being exposed a second time. A second filter which selectively absorbs and transmits a second color from the light passing through it is interposed in the optical projection path after the first utilized filter has been removed therefrom. The photoconductive surface is thus discharged to record a second electrostatic latent image thereon. This electrostatic latent image then moves through a second development station which develops the second image with a second complementary colored toner. The developed image is then transferred to the support material in superposition with the developed image of the first color toner already adhering thereto.
By repeating this process a desired number of times, a color reproduction is formed on the support material. The support material then moves into a fusing area where the colored reproduction may be permanently affixed thereto. Although not so limited, three cycles of the process may be utilized whereupon the image is developed with yellow, magenta and cyan toner. In such a three color system, different color separation filters are used to subtractively produce the color reproduction. These filters may be red, green and blue.
The color xerographic process produces excellent results for the reproduction of line copy, e.g., printed characters such as letters or numerals, but presents inherent difficulties where the copy to be reproduced comprises large solid areas of high density or a continuous tone image of varying density such as a photograph. At this point, a clear distinction is to be made between the problem of xerographic reproduction of dense solid areas of an original and accurate xerographic reproduction of density gradients in the highlight and shadow regions of continuous tone originals having areas of varying densities.
The former is a development problem associated primarily with an open cascade development system which problem has been largely overcome by employing specific development techniques or by altering the charge pattern present on large areas of contiguous charge on the photoreceptor, as hereinafter discussed. The latter is partially a development problem and partially a problem inherent in a high contrast and moderate range process such as xerography caused by the inability of a given photoreceptor to sense or appreciate, and consequently reproduce, small density gradients in the highlight and shadow areas of a continuous tone original such as a photograph. It is the solution of this latter problem by extending the range and improving the tone reproduction response of the xerographic process toward which the present invention is directed.
Various techniques have been proposed in the prior art to improve solid area cascade development in the xerographic process. Briefly, the problem of solid area development is due to electric field conditions in the regions of large contiguous areas of charge present on the photoreceptor. Xerographic development in these areas may delineate only their outline, developing only in the areas where there is a differential in charge on the xerographic surface. Consequently, the centers of these areas of uniform high charge, being large solid areas of dark input, do not attract and hold xerographic toner, and thus appear white or very lightly toned on the transfer copy sheet.
Since the problem of solid area development is primarily associated with open cascade development systems, one solution to the problem has been the adoption of development techniques other than cascade such as the well known magnetic brush, powder cloud, or liquid development systems, or by the use of development electrodes as for example disclosed in U.S. Pat. No. 2,777,418 to Gundlach or U.S. Pat. No. 2,952,241 to Clark et al.
Another approach towards the solution of the problem of solid area development has been to break up the continuous charge pattern on the photoreceptor using mechanical, optical, or electrical techniques. For example, Carlson suggests in U.S. Pat. No. 2,599,542 that improved solid area coverage is obtained using an electrophotographic plate which has been etched to resemble a waffle-grid design, the depressions on the surface of which plate are filled with a photoconductive substance. Weigl in U.S. Pat. No. 3,248,216 teaches selective discharge of a charged electrostatic plate by contacting the plate with a conductive element such as a metallic gravure roller having a dot pattern provided by ridges or projections, followed by exposure of the semidischarged plate to the image. Optical techniques for improving solid area coverage by breaking up the charge area on an electrophotographic plate involve exposing the plate after charging and prior to or subsequent to imaging to a screened light source. The screen may take the form of a line or comb screen or dot pattern. The plate is selectively discharged in those areas where the light passes through the screen but retains its charge in those areas blocked by the opaque areas in the screen. Examples of optical techniques for improving solid area coverage may be found in U.S. Pat. Nos. 2,598,732, 3,121,010, 3,212,888, 3,335,003, and 3,535,036.
The use of screens consisting of alternating opaque and transparent areas positioned between the object to be imaged and the photoreceptor has also been suggested in the prior art as a means for breaking up solid area images to allow uniform development. For example, Pendry in U.S. Pat. No. 3,152,528 teaches a document screen adapted to be superimposed over the document to be copied between the document and the lens system of a xerographic copy machine. The screen comprises a transparent base material having printed thereon a plurality of opaque dots or lines which serve to break up any dark or continuous tone areas present on the document to be copied. Typical of such screens, which have been in commercial use for the past several years, are those consisting of a pattern of reflecting white dots on a transparent substrate. These dots cover about 30% of the area of the screen and are arranged in a square array with a frequency of about 60-65 dots per inch.
Because of the improved solid area coverage in xerographic copies achieved by the above techniques in shadow and middle tone areas of an original such as a continuous tone photograph, the casual observer is impressed that the process has been sensitized to the point where it can "see" and consequently reproduce not only solid areas but also density gradients in the middle tone areas of the original. However, the use of such mechanical, electrical or optical discharge techniques, or of reflecting document screens wherein the opaque patterns of the screen appear faithfully reproduced on the solid areas of output copy, does not serve to significantly extend the range of the process; that is, small input density gradients in the highlight and shadow areas of the original are not shown as concomitant changes in output density in the copy.
Donald in U.S. Pat. No. 3,627,526 suggests a masking technique for improving gray scale range in an electrophotographic process by using a partially light transmitting contact screen in contact with the photoconductive member. The patent indicates that light passing through the partially transmitting areas of the screen will discharge that portion of the photoconductor to a lesser degree than adjacent light passing through the transparent areas of the screen, resulting in a dot pattern of various shades in the various areas of the copy. These dots are not modulated in area as in conventional halftone. Changing from one screen to another or removing the screen altogether in the Donald process involves the awkward step of replacing or reprinting photoconductive surfaces.
The range of an electrophotographic system is usually defined in terms of the input exposures over which changes in output density can be observed. Range can be shown graphically using a tone reproduction curve (TRC) wherein input density expressed in terms of log .sub.10 (100/Ro) is plotted against output density expressed in terms of log .sub.10 (100/Rc), where Ro is the percent spectral reflectivity of the original and Rc is the percent spectral reflectivity of the copy. Thus, where the reflectivity approaches 100% (white areas), the density approaches 0 (log .sub.10 100/100=0); where the reflectivity decreases, (colored areas), the density increases. For example, at 10% reflectivity, the density is 1; at 1% reflectivity, the density is 2. A typical TRC of solid area xerography embodying a selenium photoreceptor plotted over a plurality of input densities is shown as the solid curve in FIG. 1. For the purposes of the present invention, the range is defined as the density differential on the abscissa axis between points where the slope of the "S" shaped TRC is 0.5. The range of the system shown in FIG. 1 is about 0.6.
The TRC in FIG. 1 illustrates clearly why normal xerographic systems have a limited capability in reproducing pictorial originals. Opaque photographs typically have a density range in the order of about 1.5 (Dmax = 1.6: D min = 0.1) and simply cannot be accurately reproduced by a system with a range of 0.6. Varying the exposure above or below the point where the minimum output density occurs for an input density of zero serves merely to shift the TRC with no range extension and at the cost of sacrificing shadow or highlight information. In fact, range extension can be achieved only by "flattening" the TRC curve to approach as nearly as possible the dotted straight line of FIG. 1 which represents the optimum faithful reproduction of all densities.
Compound document screens composed of a mixed dot pattern of absorbing black dots and reflecting white dots have been proposed for extending xerographic range in both color and conventional black and white xerography, as disclosed in copending application Ser. No. 408,707, filed Oct. 23, 1973 now U.S. Pat. No. 3,905,822. While screens of the type disclosed in the copending application are suitable for use in color electrophotography, they are relatively inflexible with regard to affecting the color balance or hue rendition of the copy in a color process. Also, it is found that the halftone dot pattern in the color copy produced by a color process such as recited above consists of overlapping dots consisting of a mixture of the three subtractive primary colors. Thus, for example, copies of colored originals made on a Xerox brand 6500 color copy machine using such a document screen are found to contain cyan, magenta and yellow halftone patterns with the dots of each color lying on top of one another. This situation gives rise to a significiant moire problem in the color process and a disadvantage in the perceptual sensation of color saturation.
Accordingly, it is an object of the present invention to provide a simple and economical means for improving the range capabilities of high contrast and moderate or low range electrophotographic processes.
Another object is to provide a simple screening technique for use in a color electrophotographic method which both extends the range of the process and reduces moire.
A more specific object is to extend the range of a color xerographic process whereby a xerographic halftone image of improved color purity and saturation is formed.